<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/111216>111216</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Deduction guide fails for CTAD for template aliases
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          agraham-roku
      </td>
    </tr>
</table>

<pre>
    Here's a case where CTAD for template aliases feature isn't working. Minimal code is copied below, but here also are godbolt links:
* https://godbolt.org/z/6jdo4qGG9 clang-trunk doesn't work
* https://godbolt.org/z/3KoG4PfWT GCC supports this, as far back as 12.1
```
#include <chrono>

template <typename Rep, typename Period>
struct duration {
    constexpr duration(Rep r) : v{r} {}

    template <typename R2, typename P2>
    constexpr duration(duration<R2, P2> const& d)
            : v{std::chrono::duration_cast<decltype(v)>(d.v)} {}

 std::chrono::duration<Rep, Period> v;
};

template <typename Rep, typename Period = std::ratio<1>>
using seconds = duration<Rep, Period>;

template <typename Rep> using milliseconds =
 seconds<Rep, std::chrono::milliseconds::period>;

template <typename Rep, typename Period, typename Period2>
duration(duration<Rep, Period>) -> duration<Rep, Period2>;

constexpr auto d1 = seconds{1};
constexpr auto d2 = milliseconds{d1};
static_assert(d2.v.count() == 1000);

int main() {}
```
The clang error is:
```
<source>:21:1: error: deduction guide template contains a template parameter that cannot be deduced
   21 | duration(duration<Rep, Period>) -> duration<Rep, Period2>;
      | ^
<source>:20:51: note: non-deducible template parameter 'Period2'
   20 | template <typename Rep, typename Period, typename Period2>
      | ^
1 error generated.
Compiler returned: 1
```
</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJysVl-P4jYQ_zTmZbQoGQOBhzywYdlKVaXTaaU-nhx7AN86dmo7bK-fvnIS_izHnu7UIiCOPfOb3_zx2CIEvbdEJZs_svlmIrp4cL4Uey8Oonnw7rWb1E59K38jTwyLAAKkCARvB_IE1ct6AzvnIVLTGhEJhNEiUIAdidh5Ah0swyLCm_Ov2u6n8Ie2uhEGpFNpFaRrNSmoybg3hhXUXYQeW5jgQHiCvVO1MxGMtq-B8TXLNixbM1zDIca2n8Etw-0oN3V-z3D7D8Pt4qtys7-en1cgjbD7h-g7-wrK0RWpn0fjv7vn2afdny_wXFUQurZ1PgaIBx0ScRFgJzzUQr6mcY7TfMReZON3NMW1laZTBIxX8uCddYw_jYv9_zmajFfxW0tWNASfqU1mzu-fyGunzpoh-k5GUJ0XUTsLrHgcFgAApLMh0t-tP68zXH6mFjzDFTC-hiMrHj0rNr1esblmkwDuM8L3hPBM5kOb5yGvBu1eaZBluADFcHWBOH1OBENUKT98fYpaGp8gv0gRIuOVImkSJ4bLY0LjT8nutB_f9--HsInpEPlzwOHI-BjcBHUe_2rugPHNxXhvjvEqT4xPgeyCtnsIJJ1VoZf_Aa-fZcKfYMBttDH6CvwUj2HmYuFugK6Vh5n2V4ncKefvpy5V9UEd3UYBV_CQfPxIBr9neKlV0UUHKh9SMzpXPObXeb4Vxl74XTiKR_VOJUQRtfwiQiAfE32cHqfSdTa9DFtwk0DyLMv6mn3HTtsIjdD2JHtdwe97y8uBhlYH5L3zoK_65U0X4lVwnZfUB2ONOePr9BsU00CR6mTfSvadVnRpAdLZKLRNR8F5rhVeNBTJQzyICFJY6yLUNKCQOu9pzIEV1Uct4T-lcuwVRQVs_nTfy4zx9bx307pIw9M-9Bx1beiePwyLky0sLm5kvaH_q7LvUM_HFO7JkheR1HSYr1zTakMePMXOW0o7E25PmokquVrxlZhQmRe4zIrlYrWcHMq8IFEsZ7lYFiiyFZf1Lp_3ArtsVdNyokvMcJZn2Qzzec6XU0RVCxQy2-045osFm2XUCG2mxhybdDpOdAgdlXmeY76YGFGTCf11AtHSG_SrDDHdLnyZlB7qbh_YLDM6xHCBiToaKjc3VbcT2oT-ivHhXWPSeVPeHN46Hrp6Kl3DcJssjI-H1ruvJCPDbc8rMNyOxI8l_hsAAP__x4fAgg">